This invention deals with colored nacreous pigments and with processes for producing these pigments.
Nacreous pigments produce pearl-like, metallic, and iridescent effects. A widely used type comprises muscovite mica platelets coated with a metallic oxide, such as titanium dioxide. A relatively thin titanium dioxide coating produces a pearl-like or silvery luster. Mica platelets with thicker coatings produce color, even though the components are colorless, through the phenomenon of light interference; they are known as interference pigments. The color, called the reflection color, is seen most effectively by specular or mirror-like reflection, where the angle of reflection equals the angle of incidence. The reflection color is a function of optical thickness, i.e. the geometrical thickness times the refractive index, of the coating. Optical thickness of about 80 nm to about 140 nm produce reflections which may be called white, silvery or pearly; optical thickness of about 190 nm or more produce colored reflections.
Combination pigments are more complex. The oxide-coated mica pigment is further coated with an absorption pigment or dye, so-called because it absorbs some portion of the visible spectrum. If the absorption colorant has the same hue as the reflection color of the oxide-mica pigment, that color is intensified and is seen over a wide range of angles; if it has a different hue, the reflection color or a color close to it is seen at the specular angle, whereas the hue of the absorption pigment is seen at other angles. In some cases, transition colors may be seen between the extremes. Thus a single pigment has more than one color. The absorption colorant coat should be uniform and should adhere firmly to the oxide-coated mica particles.
In known combination pigments, the desired results are achieved by depositing the colorant or a precursor on the pigment platelets from aqueous solution. For example, U.S. Pat. No. 4,309,480 teaches that iron blue (ferric ferrocyanide) may be precipitated onto TiO.sub.2 -coated mica by the reaction of ferric chloride and potassium ferrocyanide in aqueous solution. Aluminum hydroxide may be precipitated after the iron blue or simultaneously with it, but it is not required for the formation of the iron blue coating. U.S. Pat. No. 3,951,679 shows that an Fe(II) phosphate layer may be precipitated onto mica pigments from aqueous solution and then converted in place to ferrous ferrocyanide by reaction with ferrocyanide solution, followed by oxidation in place to ferric ferrocyanide. U.S. Pat. No. 4,084,983 describes the formation of colored lakes on mica pigments by first depositing aluminum hydroxide on the surface from soluble reactants and then reacting with a dye in solution
Many colored pigments of very desirable properties cannot be formed from a water-soluble reactant or reactants. In the present invention, such insoluble pigment particles are dispersed in water and then deposited on the surface of the mica platelets to form continuous, adherent, and smooth colored coatings, creating a new class of combination pigments.
It is an object of the present invention to make combination pigments containing absorption pigments which are not soluble in water and which cannot be formed in place from a water-soluble reactant or reactants. Examples are pigments like the phthalocyanines, quinacridones, perylenes, dioxazines, and carbon black. Although insoluble in water, these pigments can be dispersed in water. They have high color intensity, lightfastness and bleed resistance, properties which make the resulting combination pigments suitable for automotive finishes as well as for general use in paints and in plastics incorporation. They are also useful in cosmetics when cosmetically acceptable components are employed.